1. Field of the Invention
The present invention relates to an optical polarization conversion system applicable to projection TVs and monitors, more particularly, to a polarization conversion system constructed of a polarization conversion plate coupled to a reflection type polarizing plate having a metal grating pattern.
2. Discussion of the Background Art
Along with the scale-up trend of TVs, more researches are now focused on a projection system.
A typically used image element for the projection system includes a liquid crystal deposited on polysilicon and silicon crystal, or a nano-technique based DMD (Digital Micromirror Display).
Particularly, a projection system using the LCD displays a screen by modulating the polarization of light. In other words, when it is necessary to irradiate a light to a liquid crystal, the light should be polarized. To do so, a lot of elements are needed to convert unpolarized light to polarized light.
One of the elements is a reflection type polarizer. FIG. 1 illustrates one embodiment of such reflection type polarizer.
A reflection type polarizing plate in FIG. 1 acts as a diffraction grating if a gap formed on the polarizer is greater than a wavelength of incident light. And if the gap is smaller than a wavelength, the reflection type polarizing plate goes back to its role as a polarization plate. Taking advantage of these features, an aluminum line is implanted on a glass substrate at a frequency smaller than the wavelength.
In addition, the reflection type polarizing plate in FIG. 1 transmits a polarized light whose electric field vertically vibrates to the grating, and splits or isolates a polarized light whose electric field horizontally vibrates to the grating.
Among traditional polarization conversion systems, there is one using a Polarization Beam Splitter (hereinafter, it is referred to as ‘PBS’) array. The PBS array is an element that converts unpolarized light emitted from a light source to one type of polarized light. The PBS, using specific lamination between two glass members, transmits P-waves and reflects S-waves.
The PBS array is a multi-layered PBS.
FIG. 2 shows one embodiment of an optical system to which the PBS array is applied. As depicted in the drawing, there is a fly-eye lens 201 uniformly splitting a light beam incidented on a front end of the PBS array 202 and at the same time converging the split beams. This fly-eye lens array is what creates on the PBS array 202 bright and dark portions, namely a portion where the light is incident and another portion where the light is not incident.
To polarize light in one direction only, a λ/2 plate 203 is provided to an emission surface of the PBS positioned at a portion where light is incident, and no λ/2 plate 203 is provided to the PBS positioned at a portion where light is not incident.
The λ/2 plate 203 is a polarization conversion plate converting the direction of linear polarized light. For example, it converts S-waves (i.e. vertical linear polarized light) to P-waves (i.e. horizontal linear polarized light), and P-waves to S-waves.
For instance, suppose that the polarization beam splitters in the PBS array are manufactured to transmit P-waves and reflect S-waves. Therefore, when P-polarized light incidents upon the PBS array 202 through the fly-eye lens 201, the P-polarized light transmits a corresponding PBS and eventually is incident on the λ/2 plate 203. The λ/2 plate 203 then converts the P-wave to S-wave and emits. This converted S-wave travels via a relay lens 204 and is emitted to a panel 205.
On the other hand, when S-polarized light incidents upon the PBS array 202 through the fly-eye lens 201, the S-polarized light is reflected by a corresponding PBS and incidents on a PBS without the λ/2 plate 203. This PBS without the λ/2 plate 203 again reflects the reflected incident S-wave and emits it to the panel 205. As a result, only S-polarized light incidents upon the panel 205.
Also, even though unpolarized light may be incident on the PBS array 202 through the fly-eye lens 201, a PBS with the λ/2 plate 203 transmits only P-waves to the λ/2 plate 203, and reflects S-waves onto a PBS without the λ/2 plate 203. As such, S-waves being converted at the λ/2 plate 203, and S-waves being reflected from the PBS without the λ/2 plate 203 are emitted to the panel 205.
However, the above-mentioned PBS works best only for light received at a fixed angle, and loses its role if light-receiving angle is changed even a little bit. The same problem is found in the PBS array also.
As another matter, PBS glasses are easily heated up as the intensity of light is increased. Hot glasses in turn cause stresses and these stresses generate problems like photoelasticity and non-uniform refraction index.
That is, as the temperature of a PBS lamination surface is increased, photoelasticity phenomenon occurs on glass material or refraction index is changed, so polarization itself gets lost. Moreover, with the current techniques, an allowable F/# is hardly greater than F/2.3.